Although the use of such bearings is common in devices incorporating small electric motors, such as disc drives, preload force variations in such bearings are difficult to control in practice. Variations that reduce the preload force can cause play between the rotating and stationary members and/or undesired oscillations. Variations that increase the preload force can cause other problems, such as excessive or uneven wear in the bearings and/or balls.
Although some control mechanisms exist within the systems that apply the preload force, the need for spindle bearings having an internal control mechanism remains to be satisfied.